This proposal is directed at three aspects of neural control of the circulation: effects of hypoxemia, regulation of cerebral blood flow, and control of blood flow in vasa vasorum. We plan to continue studies concerning compensatory mechanisms which help to sustain whether neural stimuli, such as stimulation of low-pressure baroreceptors, increase the sensitivity of chemoreceptors to hypoxia in man. Several studies will be directed at neural control of cerebral blood flow. Cerebral vessels are well innervated, but it has been difficult to establish the functional signficance of the innervation. We have used labeled microspheres to measure cerebal blood flow. This technique circumvents the methodologic problems created by the presence of multiple arteries and veins to and from the brain, and appears to be a valid technique for measuring both total and regional cerebral blood flow. We will compare this technique with the Xenon clearance technique in measurement of blood flow. We will examine the effects of adrenergic nerves on cerebral vessels. In these studies we will attempt to unmask an effect of sympathetic nerves on cerebral vessels by examining responses of large cerebral arteries. Further studies will be directed at determining whether cerebral vascular responses to increases in intracranial pressure are mediated in part by sympathetic nerves. Cerebral vessels are reported to have extensive cholinergic innervation: studies are planned to determine whether the cholinergic innervation is important functionally. Studies are also planned to examine control of blood flow in vasa vasorum. Labeled microspheres will be used to estimate blood flow to vessels. We plan to evaluate neurohumoral control of vasa vasorum in the aorta, smaller arteries, and veins.